The use of solid state drives (SSDs) in data storage systems is increasing. SSDs are advantageous over hard disk drives (HDD), for example, for a variety of reasons, including but not limited to: SSDs have faster random access time; SSDs have lower read latency time; SSDs have more consistent read performance; defragmentation is not necessary for SSDs; SSDs have no moving parts and thus, make no noise; SSDs do not require special cooling system and can tolerate higher temperatures; SSDs are smaller and lighter; SSDs generally are more reliable and have longer lifetimes; and SSDs consume less power. A significant drawback of SSDs, however, is their relatively high cost compared to HDDs.
SSDs can be based on either volatile (RAM-based) or non-volatile (flash) memory. Non-volatile memory can maintain information even when there is no power, whereas volatile memory requires continual power to maintain stored information. SSDs having non-volatile flash memory store information in memory cells that are made from floating-gate transistors. Memory cells can be either NOR gate flash or NAND gate flash. NOR allows for random-access reading and NAND allows only page access.
SSDs having non-volatile flash memory traditionally are either single-level cell (SLC) devices or multi-level cell (MLC) devices. SLC devices store only one bit of information per cell, and thus each cell can have one of two states (i.e., 0 or 1). MLC devices can store more than one bit of information per cell, with most traditional MLC devices storing two bits of information per cell, and thus each cell having four possible states (i.e., 00, 01, 10, or 11). SLC devices can have advantages over MLC devices in faster write speeds, lower power consumption, and higher cell endurance. However, SLC devices generally cost more per megabyte of storage than MLC devices.
In view of the foregoing, there is a need for an improved SSD having the beneficial features of both SLC and MLC technology. More particularly, there is a need for hybrid solid state drives having both single-level cells and multi-level cells and data storage systems including such solid state drives.